1. Field of the Invention
This invention relates to single viewpoint photogrammetry used to measure aircraft or wind tunnel model control surface deformation due to aerodynamic loading. More specifically, the invention is a method of correcting control surface angle measurements affected by wing bending-induced bias errors when using single viewpoint photogrammetry on an aircraft or model experiencing aerodynamic loading.
2. Description of the Related Art
A single-camera/single-view (hereinafter referred to as “single viewpoint”) photogrammetric technique is used to measure flow-induced wing twist and control surface deformation. This technique is used in wind tunnel environments for aircraft models and in-flight for actual aircraft. A typical wind tunnel test set-up is illustrated in FIG. 1 where a single camera or other imaging device 10 is positioned at a side wall of 12 of a wind tunnel test area. (For an in-flight, actual aircraft set-up, device 10 would typically be located on the aircraft's fuselage.) Coupled to camera 10 is a processor 11 for implementing single viewpoint photogrammetry computations/solutions in ways already well known in the art. See, for example, “Videogrammetric Model Deformation Measurement Technique,” A. W. Burner et al., J. of Aircr., Vol. 38, No. 4, July/August 2001, pp. 745–754, the contents of which are hereby incorporated by reference, as if set forth in their entirety. For in-flight measurements on actual aircraft see, for example, “Aeroelastic Deformation: Adaptation of Wind Tunnel Measurement Concepts to Full-Scale Vehicle Flight Testing,” A. W. Burner et al., paper presented at NATO AVT-124 Specialists Meeting, Budapest, Hungary, April, 2005, pp. 9-1 to 9-17.
Camera 10 is focused through a window 12A of wall 12 on a portion (e.g., a wing) of an aircraft or aircraft model 14 positioned in the wind tunnel. Aircraft 14 has a fuselage 16 and wings 18. In evaluating wing twist and control surface deformation, camera 10 would be focused on a wing that has visual targets (not shown) placed thereon.
In FIG. 1, aircraft 14 is viewed from a “head on” viewpoint while camera 10 has a viewpoint from above and to the side of aircraft 14. The wind tunnel's direction of wind flow will be from the “head on” direction. A rectangular X,Y,Z coordinate system is defined and is used when generating the single viewpoint photogrammetric solutions. Typically and for purposes of this description, the following conventions will be applied:                the X-dimension is in the “head on” direction or direction of wind flow,        the Y-dimension is perpendicular to the X-dimension and is in the spanwise direction of wings 18, and        the Z-dimension is perpendicular to the X and Y-dimensions and, therefore, defines the vertical direction with respect to the camera coordinate system.        
As is well known in the art, single viewpoint photogrammetric solution generation requires that one of the three X,Y,Z coordinates must be known so that a set of two equations and two unknowns can be solved. For pitch-sweep wind tunnel testing without aircraft roll, the known coordinate is the Y-coordinate or spanwise locations of the visual targets on the wings. However, target locations change as wind flows over wings 18. That is, as air flows over wings 18, aerodynamic forces act on the wings and tend to cause them to bend (typically upward for rearward swept wings with a positive load) as indicated by dashed lines 20. (Note that the amount of bending has been exaggerated for purposes of illustration.) The resulting Y-shift for visual targets (not shown) used to determine angles on the main wing surface at a given spanwise station are very similar. Thus, the effect of wing bending-induced bias error for visual targets on the main wing element is typically less than 0.1° for the worst case of near the wing tip. However, the differences in Y-shift can be considerable (leading to significant wing bending-induced bias error) for targets on a control surface that is angled with respect to the main wing surface as will now be explained with the aid of FIGS. 2 and 3.
In FIG. 2, aircraft wing 18 is illustrated in cross-section in the X-Z plane with a main wing 18A and a control surface 18B. Control surface 18B is angularly disposed with respect to main wing 18A and, therefore, the direction of wind flow in the X-dimension. Visual targets 22 and 24 are applied to control surface 18B with targets 22 and 24 being separated by a distance “d”. Targets 22 and 24 are positioned in the same Y-location for this illustration.
FIG. 3 is a Y-Z plane view of wing 18 and depicts what happens to targets 22 and 24 in the Y-dimension as wing 18 bends during a “wind on” condition. With no wind or “wind off”, targets 22 and 24 have the same Y-location as mentioned above. However, with “wind on”, control surface 18B along with main wing 18A bends as depicted by dashed line 20. As a result, the Y and Z coordinates of targets 22 and 24 change and are shifted with respect to one another. This condition results in significant error in single viewpoint photogrammetric solutions generated for control surface 18B.